Remove nullary_exprt::move_to_operands deletion

[thomasspriggs]

There are no other instances of move_to_operands across the code base and no implementation here. Therefore this removal is clean-up assuming that this member function has been removed from everywhere else and this instance has accidentally been left behind.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[tautschnig]

Thank you, apparently I forgot about this in d5a1840.

[codecov]

Codecov Report

Merging #6646 (afc53d8) into develop (b17a993) will increase coverage by 0.01%.
The diff coverage is 98.26%.

@@             Coverage Diff             @@
##           develop    #6646      +/-   ##
===========================================
+ Coverage    76.72%   76.73%   +0.01%     
===========================================
  Files         1579     1579              
  Lines       181938   181999      +61     
===========================================
+ Hits        139587   139652      +65     
+ Misses       42351    42347       -4     

Impacted Files	Coverage Δ
.../goto-instrument/goto_instrument_parse_options.cpp	69.24% <ø> (ø)
src/goto-instrument/unwind.h	100.00% <ø> (ø)
src/util/std_expr.h	93.03% <ø> (ø)
src/util/arith_tools.cpp	85.63% <50.00%> (+3.19%)	⬆️
src/util/simplify_expr_boolean.cpp	91.15% <97.43%> (+2.26%)	⬆️
src/analyses/goto_check_c.cpp	90.81% <100.00%> (+0.17%)	⬆️
src/ansi-c/ansi_c_declaration.cpp	74.07% <100.00%> (ø)
src/goto-instrument/cover_basic_blocks.cpp	90.76% <100.00%> (+0.07%)	⬆️
src/goto-instrument/unwind.cpp	100.00% <100.00%> (ø)
src/goto-programs/builtin_functions.cpp	56.86% <100.00%> (+0.05%)	⬆️
... and 5 more

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